Illumination optical units for metrology systems are known for example from DE 10 2011 084 255 A1, WO 2011/144 389 A1, WO 03/096 356 A2 and DE 102 20 815 A1. A mask inspection system including an illumination optical unit is furthermore known from WO 2005/045 503 A1. Illumination optical units for projection exposure apparatuses are known from US 2012/0 236 284 A1, U.S. Pat. Nos. 8,094,290 B2, 8,144,308 B2 and US 2012/0 249 989 A1.